The present disclosure relates generally to glass sheets and more specifically to a glass soot deposition and sintering process for forming uniform glass sheets having a desired composition and coefficient of thermal expansion.
Glass sheet materials can be formed using a variety of different methods. In a float glass process, for example, a sheet of solid glass is made by floating molten glass on a bed of molten metal. This process can be used to form glass sheets having uniform thickness and very flat surfaces. However, float glass processes necessarily involve direct contact between the glass melt and the molten metal, which can lead to undesired contamination at the interface and less than pristine surface quality. In order to produce high quality float glass sheets with pristine surface properties on both major surfaces, float glass is typically subjected to one or more surface polishing steps. This processing adds additional expense. Moreover, it is believed that the float process has not been used to make rollable (i.e., very thin) glass sheets.
An additional method for forming glass sheet materials is the fusion draw process. In this process, molten glass is fed into a trough called an “isopipe,” which is overfilled until the molten glass flows evenly over both sides. The molten glass then rejoins, or fuses, at the bottom of the trough where it is drawn to form a continuous sheet of flat glass. Because both major surfaces of the glass sheet do not directly contact any support material during the forming process, high surface quality in both major surfaces can be achieved.
Due to the dynamic nature of the fusion draw process, the number of glass compositions suitable for fusion draw is limited to those that possess the requisite properties in the molten phase (e.g., liquidus viscosity, strain point, etc.). Finally, the apparatus used in the fusion draw process can be expensive.
Thin glass sheets having a coefficient of thermal expansion (CTE) closely matched to silicon can be formed by conventional float and fusion draw processes. It is impractical, however, to modify the glass composition in these processes in order to achieve a particular coefficient of thermal expansion in the resulting glass, even assuming a particular glass composition was determined to be fusion drawable. Notably, it would be time consuming and expensive to reconfigure the fusion draw process apparatus, including the melt tanks, fusion isopipes and glass conveyance equipment for each new glass composition. Moreover, the potential for cross-contamination issues make it difficult to even consider routinely changing the glass composition in a fusion draw process in order to produce thin glass sheets having a desired coefficient of thermal expansion.